Moving the mouse handling code into a separate thread


(In a puny attempt to write my SoC project progress to my mentor, I decided to expand it and share my thoughts with you)

Today, we have two methods to register the pointer devices on Xorg server: (1) under SIGIO and (2) put they fd on EnableDevices set. There is also the silken mouse concept, which means updates fired during sigio handler (in the case of hw cursor).

We always try to prioritize silken - i.e. when the device emits a move event, it will be “painted” on the screen and the WaitFor loop still continue sleeping on select() - But the problem with SIGIO is that it can blocks if the main thread is wedged doing kernel stuff (like paging). It can’t interrupt a program in D state.

So, the basically idea is to do a separate thread which takes care the mouse handling code without using SIGIO. I did an approach and some questions were raised up:

(1) With (silken/hw cursor) or without the input thread seems to be equal in perfomance (tested with three video cards: ATI Rage XL, GeForce FX 5500 and GeForce2 MX/MX 400). I’d tested with a gnome-session started and ran ‘x11perf -putimagexy500’. The cursor never lag the mouse in both situations. At least no performance regressions :) Fine.

(2) But I think that (1) is not the exactly problem which we’re trying to solve. Daniel Stone said once to me that having a tiny footprint that needs to be kept in memory, it wouldn’t need to wait to be paged into the active set all the time. Here Daniel’s transcription: “Currently it works almost like this, but SIGIO is in the same process, with a very large memory footprint. So if any part of the X server is waiting to be paged in to memory, then you’ll be completely blocked on disk I/O. This is the problem we have today: under heavy disk and memory loads, we end up blocked on I/O. OTOH, the input thread won’t get paged out, because its active set will be extremely small”. But how to keep this resident? Is it inner to the thread?

(3) Another thing that is breaking my head is to not have such a mechanism to do a real performance test. How to know if the thread has advanced or not the overall performance? Maybe using the ‘time’ tool? Maybe something with xtest? I don’t know.

(4) So far I’m not facing any problem concerning the thread safety. Yesterday, on the IRC, Mercury and Clee tell me to test the input thread on SMP machines to really do it parallelized. I haven’t done it yet. Some another tips here?

The input thread (using clone syscall) is on my Linux machine. The patch applies with the last git evdev and xserver. You can see it here:

I’ll really appreciate comments.